Acquired (adaptive or specific) immunity is not present at birth. It is learned. As a person's immune system encounters foreign substances (antigens), the components of acquired immunity learn the best way to attack each antigen and begin to develop a memory for that antigen. Acquired immunity is also called specific immunity because it tailors its attack to a specific antigen previously encountered. Its hallmarks are its ability to learn, adapt, and remember. Acquired immunity takes time to develop after initial exposure to a new antigen. However, because a memory is formed, subsequent responses to a previously encountered antigen are more effective and more rapid than those generated by innate immunity.

Lymphocytes are the type of white blood cell responsible for acquired immunity. Typically, an acquired immune response begins when antibodies, produced by B cells (B lymphocytes), encounter antigen. Dendritic cells, cytokines, and the complement system (which enhances the effectiveness of antibodies) are also involved.

Lymphocytes

Lymphocytes enable the body to remember antigens and to distinguish self from nonself (foreign). Lymphocytes circulate in the bloodstream and lymphatic system and move into tissues as needed.

The immune system can remember every antigen encountered because, after an encounter, some lymphocytes develop into memory cells. These cells live a long time—for years or even decades. When these cells encounter an antigen for the second time, they recognize it immediately and respond quickly, vigorously, and specifically to that particular antigen. This specific immune response is the reason that people do not contract chickenpox or measles more than once and that vaccination can prevent certain disorders.

Lymphocyte

Lymphocytes may be T cells or B cells.

T Cells: T cells are produced in the thymus. There, they learn how to distinguish self from nonself. Only the T cells that ignore self antigen molecules are allowed to mature and leave the thymus. Without this training process, T cells could attack the body's cells and tissues.

Mature T cells are stored in secondary lymphoid organs (lymph nodes, spleen, tonsils, appendix, and Peyer's patches in the small intestine). These cells circulate in the bloodstream and the lymphatic system. After they first encounter a foreign or abnormal cell, they are activated and search for those particular cells.

There are different types of T cells:

• Killer (cytotoxic) T cells attach to particular foreign or abnormal (for example infected) cells because they have encountered them before. Killer T cells may kill these cells by making holes in their cell membrane and injecting enzymes into the cells or by binding with certain sites on their surface called death receptors. This binding triggers reactions within the foreign or abnormal cell that lead to death.
• Helper T cells help other immune cells. Some helper T cells help B cells produce antibodies against foreign antigens. Others help activate killer T cells to kill foreign or abnormal cells or help activate macrophages enabling them to ingest foreign or abnormal cells more efficiently.
• Suppressor (regulatory) T cells produce substances that help end the immune response or sometimes prevent certain harmful responses from occurring.

Sometimes T cells—for reasons that are not completely understood—do not distinguish self from nonself. This malfunction can result in an autoimmune disorder, in which the body attacks its own tissues (see Autoimmune Disorders).

B Cells: B cells are formed in the bone marrow. B cells have particular sites (receptors) on their surface where antigens can attach.

The B-cell response to antigens has two stages:

• Primary immune response: When B cells first encounter an antigen, the antigen attaches to a receptor, stimulating the B cells. Some B cells change into memory cells, which remember that specific antigen, and others change into plasma cells. Helper T cells help B cells in this process. Plasma cells produce antibodies that are specific to the antigen that stimulated their production. After the first encounter with an antigen, production of enough of the specific antibody takes several days. Thus, the primary immune response is slow.
• Secondary immune response: But thereafter, whenever B cells encounter the antigen again, memory B cells very rapidly recognize the antigen, multiply, change into plasma cells, and produce antibodies. This response is quick and very effective.

Antibodies

When a B cell encounters an antigen, it is stimulated to mature into a plasma cell or a memory B cell. Plasma cells then release antibodies (also called immunoglobulins, or Ig). Antibodies protect the body in the following ways:

• Helping cells ingest antigens (cells that ingest antigens are called phagocytes)
• Inactivating toxic substances produced by bacteria
• Attacking bacteria and viruses directly
• Activating the complement system, which has many immune functions

Antibodies are essential for fighting off certain types of bacterial and fungal infections. They can also help fight viruses.

Basic Y Structure of Antibodies An antibody molecule is basically shaped like a Y. The molecule has two parts: Variable part: This part varies from antibody to antibody, depending on which antigen the antibody targets. The antigen attaches to the variable part. Constant part: This part can be one of five structures, which determines the antibody's class—IgM, IgG, IgA, IgE, or IgD. This part is the same within each class.

Each antibody molecule has two parts. One part varies. It is specialized to attach to a specific antigen. The other part is one of five structures, which determines the antibody's class—IgM, IgG, IgA, IgE, or IgD. This part is the same within each class and determines the function of the antibody.

IgM: This class of antibody is produced when a particular antigen is encountered for the first time. The response triggered by the first encounter with an antigen is the primary immune response. IgM then attaches to the antigen, activating the complement system and making the antigen easier to ingest.

Normally, IgM is present in the bloodstream but not in the tissues.

IgG: IgG, the most prevalent class of antibody, is produced in greater amounts when a particular antigen is encountered again. More antibody is produced in this response, called the secondary immune response, than in the primary immune response. The secondary immune response is also faster and the antibodies produced—mainly IgG—are more effective. IgG protects against bacteria, viruses, fungi, and toxic substances.

IgG is present in the bloodstream and tissues. It is the only class of antibody that crosses the placenta from mother to fetus. The mother's IgG protects the fetus and infant until the infant's immune system can produce its own antibodies. Also, IgG is the most common class of antibody used in treatment.

Strategies for Attack Different types of invading microorganisms are attacked and destroyed in different ways. Some microorganisms are directly recognized, ingested, and destroyed by cells that ingest such invaders (phagocytes), such as neutrophils and macrophages. However, phagocytes cannot directly recognize certain bacteria because the bacteria are enclosed in a capsule. In these cases, B cells have to help phagocytes with recognition. B cells produce antibodies against the antigens in the bacteria's capsule. The antibodies attach to the capsule. The phagocyte can then recognize the bacteria. Some microorganisms cannot be completely eliminated. To defend against these microorganisms, the immune system builds a wall around them. The wall is formed when phagocytes, particularly macrophages, adhere to each other. The wall around the microorganisms is called a granuloma. Some bacteria thus imprisoned may survive in the body indefinitely. If the immune system is weakened (even 50 or 60 years later), the walls of the granuloma may crumble, and the bacteria may start to multiply, causing symptoms.

IgA: These antibodies help defend against the invasion of microorganisms through body surfaces lined with a mucous membrane, including those of the nose, eyes, lungs, and digestive tract. IgA is present in the bloodstream, in secretions produced by mucous membranes, and in colostrum (the fluid produced by the breasts during the first few days after delivery, before breast milk is produced).

IgE: These antibodies trigger immediate allergic reactions (see Allergic Reactions). IgE binds to basophils (a type of white blood cell) in the bloodstream and mast cells in tissues. When basophils or mast cells with IgE bound to them encounter allergens (antigens that cause allergic reactions), they release substances (such as histamine) that cause inflammation and damage surrounding tissues. Thus, IgE is the only class of antibody that often seems to do more harm than good. However, IgE helps defend against certain parasitic infections that are common in some developing countries.

Small amounts of IgE are present in the bloodstream and mucus of the digestive system. These amounts are higher in people with asthma, hay fever, other allergic disorders, and parasitic infections.

IgD: IgD is present mainly on the surface of immature B cells. It helps these cells mature. Small amounts of these antibodies are present in the bloodstream. Their function in the bloodstream, if any, is not well understood.